Pneumatic radial tires having a belt in which metallic cords are arranged and which is disposed in the tread portion have been conventionally attached to high performance passenger cars, and high-speed durability, high-speed driving stability and steerability have been accomplished owing to a strong hooping effect of the belt. The conventional speed range from 100 to 200 km/h, however, has been accelerated these days, and some passenger cars to be driven at 200 km/h or more, and sometimes at 300 km/h, have been introduced. In the field of racing cars, machines which are driven at such a speed range already exist, but the driving is only under limited conditions, so that the tires mounted on them have been manufactured by sacrificing general performance requirements in ordinary driving, for example, wearing resistance and fuel economy. Hence, these tires are unable to withstand general use.
If conventional radial tires having a metallic cord belt are used in the high-speed driving as mentioned above, their tread portions are deformed by centrifugal force and repeated distortions are generated in the tread portion. This deformation appears as various phenomena depending on the size, structure and materials of the tire, for example, as a lifting phenomenon in which the outer diameter at the shoulder portion increases, a phenomenon in which the outer diameter at the center portion increases and the radius of curvature of the tread decreases or the surface becomes an irregular curve, or a standing wave phenomenon.
Furthermore, owing to the heat generated by the repeated distortion in the tread caused by such a deformation, the temperature at the tread portion rapidly rises and the adhesion between the metallic cord surface and the rubber in the belt is broken; that is, so-called ply separation occurs. In particular, the tires used in such a high-speed driving as described above require a broad ground contact area to secure the grip on the road surface and, therefore, a flattened tire having a wide tread is employed. In such a low-profile flattened tire having a wide tread, since the occupied rate of the tread portion in the entire tire is large, the above-mentioned phenomena of deformation and heat generation tend to occur.
Use of a rubber with a small loss tangent (tan .delta.) is effective for decrease of the heat generation, but when such a rubber with a small loss tangent (tan .delta.) is employed, the grip lowers and required high motional performances are hard to obtain.
Also, decrease of the deformation at the tread portion without changing the formulation of the tread rubber can be achieved by raising the hooping effect of the belt, but if the number of plies of the belt composed of metallic cords is increased, the weight of the tire increases, thus exerting adverse effects on the high-speed property of the entire car.
To solve these problems, a band formed by arranging a multiplicity of independent cords parallel with each other in the circumferential direction of the tire, adhering with rubber, and shaping into a sheet is disclosed in Japanese Patent Publication Kokai No. 47-14805, and Japanese Patent Publication Kokoku No. 55-45402. This band, however, has at least one joint portion extending in the widthwise direction. The stiffness in the circumferential direction decreases at this joint portion, thus large distortion tends to occur to induce breakage. In addition, the joint portion may impair the uniformity of the tire. Accordingly, the band is not sufficient for use in the high speed range described above. The tire having such a band makes no trouble in ordinary speed range but in a high speed range as fast as more than 200 km/h or more than 300 km/h, it is impossible to prevent deformations caused by the centrifugal force on the tread portion and no sufficient durability is obtained.
It is hence a primary object of this invention to provide a tire having a structure capable of withstanding use in such a high speed range without sacrificing other general performances.
As a means of solving the above problems, a jointless band which is formed by winding one or a plurality of organic fiber cords spirally on the belt parallel to the central circumferential line of the tire, and a joint band in which the densities of a spirally wound cord are different between the crown portion and the shoulder portion of the tire so as to enhance the tire performances, are disclosed in Japanese Patent Publication Kokoku No. 44-17801 and Japanese Patent Publication Kokoku No. 57-61601.
But this means has the problem that it takes much time to wind the cord, thus its productivity is inferior, and since the wound cord becomes asymmetrical at the section of the tire, the uniformity such as conicity is poor.
Furthermore, upon changing the cord density, if the density is set too high, the cord surfaces contact each other, which sometimes induces breakage at that point.
It is hence another object of this invention to provide a radial tire having a superior uniformity of the tire and excellent productivity in which the breakage at the shoulder portion of the belt due to the lifting in driving is surely prevented and the contact between cord surfaces is inhibited, and its manufacturing method.
It is still another object of this invention to provide an assembly of belt, band and tread rubber which can improve the high-speed durability of the radial tire as mentioned above, and a method of and apparatus for forming the band.
Generally, in the manufacturing process of radial tires, an integrally assembled body is previously formed by winding these belt, band and tread rubber sequentially on a belt drum.
As shown in FIGS. 39, 40 and 42, conventional belt drum 300 is supported rotatably at one side on a shaft 301 and has a belt-forming surface S1 of which shape in the circumferential direction is annular and of which sectional shape in the axial direction is linear. The belt-forming surface S1 is formed by plural slats 303 each having a width of approximately 40 mm arranged radially on its entire circumference, and both ends of the slat are fastened with a pair of annular springs 302. An air bag 304 equipped with an air pipe 306 is fixed inside the slats in the radial direction by a supporting base 305. Drum outer diameter defining rings 308 are detatchably mounted on frames 307 disposed on both sides of the slats.
A conventional method of manufacturing an assembly of the belt, band and tread by using the above existing belt drum is explained below:
the drum outer diameter is expanded by inflating the air bag with air supplied through the air pipe; at this time, the outer diameter of the drum is restricted to a specified belt adhering diameter by the drum outer diameter defining ring;
a belt B is formed (see FIG. 40) by winding up a plurality of plies composed of cords which are coated with a rubber and are woven in the form of cord fabric, on the belt-forming surface S1 so that the cords of each ply cross each other at an angle of 10 to 40 degrees with regard to the circumferential direction;
a band BR is formed by further winding one ply or two plies of cords which are coated with a rubber and are woven in the form of cord fabric, on the circumferential surface of the belt B at an angle of nearly 0 degree with respect to the circumferential direction; and
a tread rubber T is wound on the radially outer side of the band BR, thus forming an assembly A into one body.
The thus formed assembly is taken out from the belt drum by using a holding means and transferred to the center of a cylindrical carcass of the tire on a tire forming drum. The carcass of tire is sequentially swollen in a toroidal shape by an air pressure so as to have its circumferential surface pressed tightly against the belt inside the assembly. The belt is furthermore pressed and attached to the tire carcass over the full width of the belt by a tread attaching roller, whereby the assembly is combined integrally with the tire carcass to provide a raw tire.
Also, in Japanese Patent Publication Kokai No. 61-51979 there is disclosed a method for forming a band by spirally and continuously winding one or a multiplicity of synthetic fiber cords in the circumferential direction on the circumferential surface of the belt formed on the belt ring, covering in a range of at least 70% of the belt width, with a pitch of 5 to 15 mm in the widthwise direction.
In this method, since the winding pitch of the cord is set as wide as 5 to 15 mm, a disadvantageous expansion of the equatorial diameter of the raw tire due to the shaping internal pressure in vulcanizing process can be effectively prevented, but thermal shrinking force of the band becomes weaker at shoulder portions than at the center portion as shown in FIG. 42. Consequently, one of the band characteristics, that is, a function to prevent the lifting phenomenon of the belt owing to the centrifugal force during running cannot be fully executed. In other words, a reinforcing effect for the failure of the belt due to the lifting while running is insufficient.
Such a problem results from the fact that the band is formed by winding on a cylindrical belt drum having a linear sectional shape in the axial direction as described above and shown in FIGS. 39, 40 and 22, in spite of the belt and band of the finished tire in the mold being convex in its sectional shape in the axial direction (final shape) as shown in FIG. 20.
That is, though the circumferential winding lengths in forming are identical at shoulder portions and the center portion of the belt, the circumferential lengths of the belt after vulcanization of tire become short at belt shoulder portions and long at the belt center portion due to the shaping (expanding) process in vulcanization. As a result, the stretch becomes large at the center portion and small at the shoulder portions of the belt, and the thermal shrinking force is lessened at the shoulder portions. Since a larger stretch is applied on the center portion of the belt, a stretch difference exceeding 2% usually occurs.
The trouble mentioned above is induced because the stretch difference appears as the differences in residual elongation and thermal shrinking force as shown in FIGS. 41 and 42. Here, the term "stretch" means percentage of finishing diameter to the winding diameter of the belt.
Accordingly, another object of this invention is to provide an assembly of belt, band and tread rubber in which the difference of the stretches between the center portion and the shoulder portions of the band is made as small as 2% or less and in order to raise the thermal shrinking force at the shoulder portion, in its turn raise a function to prevent lifting of the belt layer by which a high-speed durability can be improved, and a method of and apparatus for forming the band.